In theory, cocos2d-x could show textures in any size, but actually the maxsize of the textures is restricted by the hardware and the system.
Here I provide a texture-size restriction table on different platforms’ emulators.

platform

maxsize in pixelsr

win32

2048 * 2048

android

4096 * 4096

iPhone3

1024 * 1024

iPhone3GS

2048 * 2048

iPhone4

2048 * 2048

On the real machines, they aslo have different restrictions, here are some test results: G3 (Hero) 1024 * 1024, iPhone4 2048 * 2048.

So, for the developers, if you want to cross platforms and run your games smoothly, you should keep your texture size less than 1024 * 1024, that is the lowest restriction for most machines.

Code to get max size of texture that the device support: (might not work in simulator)